jj workspace add -r HEAD

Trap: assuming jj workspace add -r <rev> makes Git resolve <rev> from the primary worktree looks natural because Git worktrees normally start from a Git revision. Fix: jj creates an orphan worktree, populates it itself, then exports Git HEAD to the requested revision’s Git commit. A validated non-tip -r @-- workspace had detached Git HEAD exactly equal to first, not the primary worktree’s second; root() correctly leaves Git HEAD unborn at refs/jj/root.